[WIP] Standardize mouse interaction events

I have changed the drag events to use pageX/pageY because they are more common.  

There is another issue with zooming with the mouse wheel that I haven't fixed yet.  Firefox doesn't support the `mousewheel` event.  Also, the meaning of the delta values returned by browsers is different.  I would propose using https://github.com/brandonaaron/jquery-mousewheel to normalize this rather than trying do so ourselves.  Is it okay to bring in this dependency?